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Benq 5V 3A and 3.3V switching power supply for LCD panels

Chips

Conversion to car 5V power supply

Benq 5V 3A and 3.3V switching power supply for LCD panels

This power supply is available at a very low cost (about $3 including shipping) from several Ebay resellers as DC-DC step-down power module current 3A 12V to 5V 3.3V. I got my one from Geiliable Store.

I needed to power my Zaurus SL-C3200, GPS and the car ride recording camera with a strong 5V power supply, so I decided to buy this one and modify it for my needs.

Chips

It is equipped with three connectors with a lot of pins marked as ADJ and SW, but don't be confused. This power supply is not adjustable nor switchable! These pins are just an interconnection between two connectors and don't have any use on the board.

It is based on a reference design of LM2576S-5.0 with an additional 3.3V linear regulator. However input voltage of LM2576 can be up to 45V, the input capacitor is designed only for 25V. So maximum input voltage of the module is 25V.

If you want to modify this power supply to make it switchable, you need to cut the PCB around LM2576 pin 5 and solder the On/Off wire there. The output voltage varies between about 4.85V with load and 5.027V with no load (dependence on the input voltage is much smaller).

Linear 3.3V regulator compatible with AIC1084-33 then makes 3.3V from 5V. You should expect up to 58% total energy efficiency for the 3.3V power. However datasheet recommends minimal load current 10mA, my one was pretty stable even with no load. The output voltage without any load varies between 2.9993V and 3.0000V.

Conversion to car 5V power supply

As I had no use for connectors and the board with them would need larger shrinkable tube, I decided to remove them. Also 3.3V power supply was not needed and its removal decreased quiescent power consumption to 6.9mA.

First I desoldered connectors and electrolytic capacitors using a soldering iron, solder wick and a cheap vacuum plunger. SMD parts (regulator and 0.1µF ceramic capacitor) were removed on a kitchen hot plate. I just carefully measured its temperature with a cheap infrared thermometer. It was possible to move spare parts out of the board a few seconds after reaching 220°C.

Then I have soldered cables to the larger holes on the board and fastened with a few drops of hot glue.

I covered my product with a hot-shrinkable tube after making photos.

top view of PCB sie view of PCB